Sarah E. Norman

Overcoming hydrolytic sensitivity and low solubility of phosphitylation reagents by combining ionic liquids with mechanochemistry

Ionic liquids have been used in combination with ball milling on a range of chlorophosphoramidite reagents to phosphitylate nucleosides and 2-deoxynucleosides. The enhanced stability offered by the ionic liquid mediated processes combined with efficient mass transfer induced by ball milling has enabled excellent yields to be obtained even when using small dialkyl amino groups as well as the more commonly used diisopropylamino protection.

Selective synthesis of chlorophosphoramidites using ionic liquids

A range of chlorophosphoramidites have been prepared in ionic liquids and compared with material synthesised in molecular solvents. Through the use of ionic liquids as reaction media the moisture sensitivity and impurity issues hampering existing traditional synthetic routes have been eased. Not only can stock chemicals be used without purification, but the reactions may be conducted at room temperature and at high concentrations. Furthermore, reaction times are reduced and rapid addition of reagents is possible whilst retaining tight control over product selectivity. Beyond their role as reaction media, ionic liquids also present a unique storage medium for these highly moisture sensitive chlorophosphoramidites.

The electroreduction of benzoic acid: voltammetric observation of adsorbed hydrogen at a platinum microelectrode in room temperature ionic liquids

The electrochemical reduction of benzoic acid in the presence and absence of hydrogen (H(2)) has been investigated using a 10 μm diameter platinum microelectrode in four different room temperature ionic liquids (RTILs), namely [C(4)mim][NTf(2)], [C(4)mpyrr][NTf(2)], [C(4)mim][OTf] and [C(4)mim][BF(4)], versus Ag/Ag(+). In all cases, reductive voltammetry is observed, and is suggested to occur via a CE mechanism in which dissociation of benzoic acid is followed by electron transfer to H(+) ultimately forming adsorbed hydrogen. Furthermore, the adsorbed H atoms, formed from the reduction of benzoic acid, could be used to achieve the rapid hydrogenolysis of the organic compound (bis(benzyloxycarbonyl)-l-lysine) on the timescale of the voltammetric technique under moderate conditions (25 °C).

Pressure effect on vibrational frequency and dephasing of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids

Raman spectra in the range of the totally symmetric stretching mode of the [PF6](-) anion, νs(PF6), have been measured for 1-alkyl-3-methylimidazolium ionic liquids [CnC1im][PF6], for n = 4, 6, and 8, as a function of pressure at room temperature. The ionic liquids [C6C1im][PF6] and [C8C1im][PF6] remain in an amorphous phase up to 3.5 GPa, in contrast to [C4C1im][PF6], which crystallizes above ~0.5 GPa. Equations of state based either on a group contribution model or Carnahan-Starling-van der Waals model have been used to estimate the densities of the ionic liquids at high pressures. The shifts of the vibrational frequency of νs(PF6) with density observed in [C6C1im][PF6] and in [C8C1im][PF6] have been calculated by a hard-sphere model of a pseudo-diatomic solute under short-range repulsive interactions with the neighboring particles. The stochastic model of Kubo for vibrational dephasing has been used to obtain the amplitude of vibrational frequency fluctuation, , and the relaxation time of frequency fluctuation, τc, as a function of density by Raman band shape analysis of the νs(PF6) mode of [C6C1im][PF6] and [C8C1im][PF6].

A simultaneous voltammetric temperature and humidity sensor

We report the simultaneous measurement of temperature and humidity by analysing square wave voltammetric responses of two ferrocene derivatives, decamethylferrocene (DmFc) and 1,2-diferrocenylethylene (bisferrocene, BisFc) in 1-(2-methoxyethyl)-1-methyl-pyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([Moepyrr][FAP]). These two molecules produce three peaks in square wave voltammetry. Through study of the peak potentials of BisFc/BisFc(+) (vs. DmFc/DmFc(+)) and BisFc(+)/BisFc(2+) (vs. DmFc/DmFc(+)) over a temperature range of 298 K to 318 K and humidity range of 1% to 50% using square wave voltammetry, the temperature and humidity dependences of the relative peak potentials were investigated. A reliable method to calculate the humidity and temperature based on the voltammetric experiment is characterised and demonstrated.

Changed reactivity of the 1-bromo-4-nitrobenzene radical anion in a room temperature ionic liquid

Radical anions of 1-bromo-4-nitrobenzene (p-BrC6H4NO2) are shown to be reactive in the room temperature ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, ([C4mPyrr][NTf2]), by means of voltammetric measurements. In particular, they are shown to react via a DISP type mechanism such that the electrolysis of p-BrC6H4NO2 occurs consuming between one and two electrons per reactant molecule, leading to the formation of the nitrobenzene radical anion and bromide ions. This behaviour is a stark contrast to that in conventional non-aqueous solvents such as acetonitrile, dimethyl sulfoxide or N,N-dimethylformamide, which suggests that the ionic solvent promotes the reactivity of the radical anion, probably via stabilisation of the charged products.

Structure of ionic liquids with amino acid anions via neutron diffraction

The liquid structures of the ionic liquids 1-ethyl-3-methylimidazolium alaninate and 1-ethyl-3-methylimidazolium serinate are fully elucidated through the application of neutron diffraction techniques. We observe significant direct interaction between anions, particularly in the case of the serinate ionic liquid which is strongly hydrogen bonding between its hydroxyl and carboxylate groups, and is attributed the significant increase in viscosity of the neat liquid to this structural feature. Minor differences in the elucidated interactions are present between the R and S forms of the anions.

The electrochemical reduction of 1-bromo-4-nitrobenzene at zinc electrodes in a room-temperature ionic liquid: a facile route for the formation of arylzinc compounds

The electrochemical reduction of 1-bromo-4-nitrobenzene (p-BrC6H4NO2) at zinc microelectrodes in the [C4mPyrr][NTf2] ionic liquid was investigated via cyclic voltammetry. The reduction was found to occur via an EC type mechanism, where p-BrC6H4NO2 is first reduced by one electron, quasi-reversibly, to yield the corresponding radical anion. The radical anions then react with the Zn electrode to form arylzinc products. Introduction of carbon dioxide into the system led to reaction with the arylzinc species, fingerprinting the formation of the latter. This method thus demonstrates a proof-of-concept of the formation of functionalised arylzinc species.

Solvation Structure of Uracil in Ionic Liquids.

The local solvation environment of uracil dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate has been studied using neutron diffraction techniques. At solvent:solute (ionic liquid:uracil) ratios of 3:1 and 2:1, little perturbation of the ion-ion correlations compared to those of the neat ionic liquid are observed. We find that solvation of the uracil is driven predominantly by the acetate anion of the solvent. While short distance correlations exist between uracil and the imidazolium cation, the geometry of these contacts suggest that they cannot be considered as hydrogen bonds, in contrast to other studies by Araújo et al. (J. M. Araújo, A. B. Pereiro, J. N. Canongia-Lopes, L. P. Rebelo, I. M. Marrucho, J. Phys. Chem. B 2013, 117, 4109-4120). Nevertheless, this combination of interactions of the solute with both the cation and anion components of the solvents helps explain the high solubility of the nucleobase in this media. In addition, favourable uracil-uracil contacts are observed, of similar magnitude to those between cation and uracil, and are also likely to aid dissolution.

Controlled chemistry of Moisture Sensitive Reagents in Ionic Liquids

PCl3 and POCl3 are key synthetic precursors in a wide range of chemical processes, required in the synthesis of compounds ranging from bulk chemicals such as pesticides, flame retardants and plasticizers to the advanced synthetic intermediates and reagents, therapeutic precursors and metal catalysts ligands. However, whether neat or in organic solvents, these reagents and their derivatives are highly air and water sensitive and must be used under strictly anhydrous conditions, often in excess at low temperatures to allow for some level of control in order to achieve chemoselectivity and avoid side-product formation. For instance, excess of phosphitylating reagent must be used in aprotic organic solvents at low temperature under anhydrous and dissolution conditions in order to obtain mono-derivatised chlorophosphines and overcome the high chemical reactivity of the trivalent chlorophosphine’s P-Cl bonds. As such, by virtue of their chemical instability in organic media, the use of chlorophosphines in the direct phosphitylation of alcohols and amines has been limited and very difficult to control. In industry, the reagents used and available for phosphitylation reactions have been restricted to compounds such as dibenzyloxychlorophosphine, dichlorodiisopropylaminophosphite, chlorocyanoethyldiisopropyl-aminophosphite and cyanoethyl-bisdiisopropylaminophosphite. Although these reagents are commercially available and used extensively as precursors in the synthesis of many phosphoruscontaining fine chemicals, major variability can be observed in their purity and stability upon storage. As a consequence, yields of phosphitylation reactions are highly variable and thus compromise overall synthetic efficiency in particular that of automated processes such as solid phase synthesis of oligonucleotides, which require highly pure nucleoside phosphoramidites to be effective. We have established that ionic liquids can alleviate many of the issues surrounding the use and preparation of P-X containing compounds as synthetic reagents. Initially, the stability of PCl3/POCl3 in ILs and the controllable reactivity of PCl3 in this medium has been demonstrated. In addition, in the area of PCl3 manipulation, ILs was shown to permit the selective conversion of individual P-Cl bonds (Figure 1) to be performed with control not observed in molecular solvent systems. This has been particularly valuable in the formation of nucleotide phosphoramidites.